20.46. xrange(): Arithmetic progression
generator

The difference is that xrange() is a generator
(see Section 24.3, “Generators: Functions that can produce a sequence of
values”), while range() actually builds a list for its result. This
means you can use xrange() in situations where
you want to generate a large series of the values from an
arithmetic progression, but you don't have enough memory to
build that series as a list.